This invention relates to sheet materials, and, in particular, transparent sheet materials comprising an ink-receptive coating.
Polymeric sheet materials, particularly transparent polymeric sheet materials, are commonly used to prepare imaged sheets for projection onto a surface for viewing. One technique for preparing such "visual transparencies" involves depositing ink on the surface of the sheet to provide the desired image. Recently, computer-driven graphic plotting devices have become available which can quickly and precisely generate complex, graphic information. These plotters generally utilize pens or ink jets containing aqueous-based inks or solvent-based inks.
While graphic plotting devices can generate particularly attractive transparencies for visual presentation, the nature of the inks poses problems in the preparation and handling of the transparencies. For example, many sheet materials do not readily accept inks, with the result that the ink beads up on the surface of the sheet, from which it can be inadvertantly removed before it has an opportunity to dry. Other sheet materials that accept the inks to a greater degree often require an extended period of time, e.g. three minutes or more, before the surface thereof is dry enough for handling. This extended drying period greatly increases the opportunity for the image to be smeared during removal of the sheet material from the plotter or during handling and stacking of imaged sheets. Another inking problem is irregular image density, including dots at the end of a pen stroke, and severe striations resulting from the multiple, adjacent pen strokes required to form a large block of color, such as when generating a bar chart, a pie chart, or the like. Another problem is disruption of the surface of the sheet material by the pen nib during printing, i.e. clogging.
Mercer et al. U.S. Pat. No. 4,301,195 describes a transparent film having an ink receptive coating on a transparent backing. The ink receptive coating comprises a mixture of two polymers or individual layers of each polymer. One polymer is the reaction product of an epoxidized, water-insoluble, neutral rubbery polymer with a water-soluble secondary monoamine. The second polymer contains the repeating units. ##STR1## and is frequently referred to as polyvinyl alcohol.
Maekawa et al. U.K. Patent No. 2,050,866 describes an ink jet recording sheet. Their examples include one in which the backing is a polyester film, and the coating is a blend of gelatin and polyvinyl pyrrolidone further containing a high percentage of silica sol (33%). The sheet is quite hazy and unsuitable for projection.
Jaeger et al. U.S. Pat. No. 4,503,111 describes a transparent receptor film for pen plotters and ink jet printers. They teach the use of polyvinyl pyrrolidone and blends of polyvinyl pyrrolidone with a matrix forming polymer that is swellable by water, insoluble at room temperature, but soluble at elevated temperatures. Because the final coating is moisture sensitive, at high humidities, handling of the film often leaves fingerprints thereon.